U.C.L.A./VA The bald mice in row C are the same mice shown in group B, after treatment with a hair-growth compound. The mice in row A were given a placebo.Mouse researchers conducting stress hormone experiments have stumbled onto a surprising new discovery — a potential treatment for hair loss.

Scientists at the University of California, Los Angeles, and the Veterans Administration were working with genetically altered mice that typically develop head-to-tail baldness as a result of overproducing a stress hormone.

The experiment wasn’t focused on hair loss. Instead, it was designed to study a chemical compound that blocks the effects of stress on the gut. The researchers treated the bald mice for five days with the compound and then returned them to the cages, where they scampered about with several furry mice from a control group.

Three months later, the scientists went back to the cage to conduct additional experiments. They were surprised by what they saw inside — all of the mice had full heads and backs of hair. The once-bald mice, eventually identified through ear tags, were indistinguishable from their normal, furry cage mates.

Dr. Million Mulugeta, co-director of the preclinical stress biology program at U.C.L.A., said he looked inside the cage and at first wondered why the bald mice weren’t there. “I asked my colleague, ‘How come these mice aren’t distinguishable from the others?’ ” he said. “We went back to our data log, and we realized all the mice had grown hair. It was a totally unexpected finding.”

The serendipitous discovery was reported Wednesday in the online medical journal PLoS One.

Already the research is drawing a mixed response from dermatologists and hair-loss researchers. Dr. Melissa Piliang, a dermatologist at the Cleveland Clinic, cautioned that the findings of a mouse study may not be applicable to humans, but she said that the results may spur more study of the role stress might play in human hair loss.

“We’ve certainly seen patients whose hair worsened when they are under a lot of stress,” said Dr. Piliang. “But what we don’t know is whether some of this genetic hair loss is particularly affected by stress. I think it’s hopeful for future research and treatment.”

But Dr. George Cotsarelis, chairman of the dermatology department at the University of Pennsylvania School of Medicine, said any treatment developed from the research would probably be useful only for hair loss related to stress, likes that caused by one-time events, rather than as a treatment for genetic baldness.

“It’s difficult to say that it’s going to lead to a new treatment,’’ he said.

Dr. Mulugeta said he is hopeful the findings will lead to new avenues of hair-loss research. The team decided to repeat the experiment several times. Each time, bald mice treated with tiny doses of the compound for five days grew new hair in just a few weeks. In another series of experiments, the compound was injected into young mice before their hair fell out. Those mice never went bald, suggesting the compound not only has the potential to grow hair but may also prevent age-related hair loss.

The effect also appeared to persist after only one series of treatments. The scientists continued to observe the mice for four months — a long time in the two-year life span of a mouse. The new hair remained on the once-bald mice, and the mice that were treated to prevent hair loss never went bald.

The duration of the effect is important, because current hair-loss prevention remedies — including minoxidil (sold under the brand name Rogaine) and finasteride (sold as Propecia) — require regular use to maintain what is typically described as only a modest benefit.

Still, Dr. Cotsarelis cautioned that the hair growth cycles are very different in mice and humans, so one could draw only limited conclusions from the research.

Dr. Mulugeta agreed that any treatment for humans is, at best, years away. “We are at the early stage of the work,’’ he said. “We have a very strong observation, but we don’t know whether this effect could be seen in humans. How does it act? All of these things have to be worked out, but we intend to follow through.’’

The mice used in the experiments had been genetically altered to overproduce a stress hormone called corticotrophin-releasing factor, or CRF. To block the action of CRF, the researchers injected the mice with a peptide called astressin-B. The experiment was designed to measure how much the peptide would inhibit the effects of stress on the colon.

The mechanism by which the peptide triggered hair growth isn’t known, Dr. Mulugeta said. The agent appears to be acting on stress hormone receptors in the skin and near, or within, the hair follicle. “The hair follicle in these mice is inactive,’’ he said. “Something has turned on that cycle to put it back into an active phase, a hair-growing phase.’’

Dr. Mulugeta also noted that in addition to preventing hair loss, the agent affected the mice’s skin pigment, suggesting the compound may have the potential to affect hair color, including gray hair.

“The evidence to show that hair loss is not a permanent loss is shown very strongly in this study, and that by itself is very important,’’ he said. “The antagonist really triggers a mechanism that covers a lot of biological phenomena, including hair growth and hair color. We certainly intend to continue the research.’’